Anzan,
Your problem seems to me that it is jetted too lean, my Rochester B does the same thing, fresh rebuild, I had to go to a 61 jet for it to run well. I'm at sealevel so it is worse for us guys. To diagnose lean jet/power valve take your truck out and get into heavy load and wait for it to cut out/stumble then pull the choke out a teany bit, if it starts to pull harder you are too lean on your tune. BTW, I might add, the single Rochester B seems to be a little under carb'd for a 235 (56 blue flame high lift cam engine 140hp). When I had my later model mono jet on it, it had a ton more power and better drivability too.
Steve

Last edited by sstock; 05/25/2013 12:11 PM.
1953 Chevrolet 3100
261 cu inch, sm420, 3.55 rear, torque tube still,omaha orange, still 6 volt, RPO green glass, side carrier spare, all done
In the DITY Gallery
Video of the 261 running

1964 GMC 1000
305 Big Block V6, sm420, the next cab off restoration